The present invention relates generally to magnetoresistive read sensors for use in magnetic read heads. In particular, the present invention relates to a spin valve head with antiferromagnetic exchange stabilization and method for forming such a spin valve.
A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
A common GMR read sensor configuration is the GMR spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the GMR spin valve, while a magnetization direction of the free layer rotates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic layer to the pinned layer. The antiferromagnetic layer is positioned upon the pinned layer such that the antiferromagnetic layer and the free layer form distal edges of the GMR spin valve. The spin valve is then heated to a temperature greater than a Nxc3xa9el temperature of the antiferromagnetic layer. Next, a magnetic field oriented in the predetermined direction is applied to the spin valve, thereby causing the magnetization direction of the pinned layer to orient in the direction of the applied magnetic field. The magnetic field may be applied to the spin valve before the spin valve is heated to the temperature greater than the Nxc3xa9el temperature of the antiferromagnetic layer. While continuing to apply the magnetic field, the spin valve is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer. Once the magnetic field is removed from the spin valve, the magnetization direction of the pinned layer will remain fixed, as a result of the exchange with the antiferromagnetic layer, so long as the temperature of the spin valve remains lower than the Neel temperature of the antiferromagnetic layer.
The free layer of a spin valve sensor must be stabilized against the formation of edge domain walls because domain wall motion results in electrical noise, which makes data recovery impossible. A common way to achieve stabilization is with a permanent magnet abutted junction design. Permanent magnets have a high coercive field (i.e., are hard magnets). The field from the permanent magnets stabilizes the free layer and prevents edge domain formation, and provides proper bias.
However, there are several problems with permanent magnet abutted junctions. To properly stabilize the free layer, the permanent magnets must provide more flux than can be closed by the free layer. This undesirable extra flux stiffens the edges of the free layer so that the edges cannot rotate in response to flux from the media, and may also cause shield saturation which adversely affects the ability of the sensor to read high data densities. The extra flux from the permanent magnets may produce multiple domains in the free layer and may also produce dead regions which reduce the sensitivity of the sensor. The junction between the permanent magnet and the various layers must be carefully engineered to minimize the stray flux as well as to minimize the junction resistance. Also, a junction of dissimilar metals can cause unwanted strain in the sensor. The free layer will respond to the strain unless the magnetostriction is exactly zeroed. Another disadvantage of permanent magnet abutted junctions is the nature of hard magnetic materials, which are multi-domained. Variation in grain size and shape leads to a distribution of domain coercivity. Lower coercivity domains may rotate when subjected to external fields. Such a grain near the edge of the free layer could cause domain wall formation and failure.
Tabs of antiferromagnetic material or xe2x80x9cexchange tabsxe2x80x9d have also been used to stabilize the free layer of magnetic sensors. Exchange tabs are deposited upon the outer regions of the free layer and are exchange coupled thereto. Functions of the exchange tabs include pinning the magnetization of the outer regions of the free layer in the proper direction, preventing the formation of edge domains and defining the width of an active area of the free layer by preventing free layer rotation at the outer regions of the free layer.
There are several advantages to the use of exchange tabs rather than permanent magnet abutted junctions. There is no junction to produce stray magnetic flux or junction resistance. Also, the lack of a junction of abutted, dissimilar metals makes it less likely that high strain will be produced within the sensor. And control is maintained over the anisotropy of the free layer regardless of how narrow the width of the active area is made.
The use of antiferromagnetic exchange tabs in AMR type sensors has been disclosed in U.S. Pat. No. 4,663,685, entitled xe2x80x9cMAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED LONGITUDINAL BIASxe2x80x9d, U.S. Pat. No. 4,713,708, entitled xe2x80x9cMAGNETORESISTIVE READ TRANSDUCERxe2x80x9d, and U.S. Pat. No. 5,753,131, entitled xe2x80x9cMAGNETORESISTIVE DEVICE AND MANUFACTURING METHOD THEREOFxe2x80x9d. Longitudinal bias in AMR type sensors is also discussed in U.S. Pat. No. 4,785,366, entitled xe2x80x9cMAGNETORESISTIVE READ TRANSDUCER HAVING PATTERNED ORIENTATION OF LONGITUDINAL BIASxe2x80x9d.
The use of antiferromagnetic exchange tabs in spin valve type sensors has been disclosed in U.S. Pat. No. 5,206,590, entitled xe2x80x9cMAGNETO RESISTIVE SENSOR BASED ON THE SPIN VALVE EFFECTxe2x80x9d, and U.S. Pat. No. 5,949,623, entitled xe2x80x9cMONOLAYER LONGITUDINAL BIAS AND SENSOR TRACKWIDTH DEFINITION FOR OVERLAID ANISOTROPIC AND GIANT MAGNETORESISTIVE HEADSxe2x80x9d.
Not all antiferromagnetic materials are suitable for use as exchange tabs in spin valve type sensors. Materials such as MnFe and TbCo are too corrosive for head production. MnFe and similar materials also have relatively low blocking temperatures, which is undesirable because the pinned regions of the free layer may become unpinned if the temperature of the sensor is raised above the blocking temperature of the exchange tabs during operation. MnFe and similar materials also have relatively low coupling constants, which results in weaker exchange coupling and higher side readings. Materials such as Fe2O3 only increase the coercivity of the ferromagnetic layer instead of the pinning field.
The prior art does not disclose an exchange tab structure that makes use of a synthetic antiferromagnet, which provides an increased pinning field and reduced side reading. Furthermore, current methods for forming sensors with exchange tabs are inadequate. A major difficulty in manufacturing sensors with exchange tabs using conventional methods is establishing adequate exchange coupling between the exchange tabs and the free layer. Adequate exchange coupling is difficult to obtain because of the presence of a residue layer. It is often necessary to employ some removal process such as a pre-sputter etch or reactive ion etch on the residue layer to expose a clean, unoxidized surface of the free layer and to establish exchange coupling with deposited exchange tab material. A single monolayer of non-magnetic residue material is sufficient to destroy the exchange coupling. The removal step is problematic, though, because a photo mask is typically present on the residue layer during the removal step. It is extremely difficult to remove residue close to the photo mask, and excess residue is often left near the photo mask. The excess residue near the photo mask prevents proper exchange coupling between exchange tab material and the free layer, which results in widening of the effective width of the sensor and generation of an off-track signal.
The presence of a photo mask causes another problem during the deposition of the exchange tab material. Near the edge of the photo mask, some amount of shadowing is inevitable, causing reduced thickness of exchange tab material near the photo mask. Thus, even if it were possible using conventional techniques to completely remove the residue near the photo mask, the reduced thickness of the exchange tab material near the photo mask would still cause a problem. Since the pinning strength of exchange tabs is strongly dependent on thickness, it is necessary to compensate for the reduced thickness near the photo mask by increasing the overall layer thickness to provide a margin of safety. As the size of recording heads shrink, thinner layers become more attractive. Collimated deposition techniques can be employed to avoid thinning, but such an approach adds an extra constraint to the deposition process. It would be preferable to optimize for material quality with no geometry constraints.
Another issue that arises in fabricating spin valve sensors using conventional techniques is the matching of magnetic flux at the sensor edge. A pre-sputter etch to remove residue will necessarily remove magnetic material from the free layer, thereby reducing the moment of the free layer and creating a mismatch of magnetic flux. The removed material can be replaced by an in situ deposition, but the deposition can never be perfect, especially in the vicinity of a photo mask.
Thus, there is a need for a method of forming a spin valve sensor with exchange tabs, which results in better exchange coupling between the exchange tabs and the free layer of the spin valve.
A spin valve sensor with antiferromagnetic exchange stabilization and method for forming such a sensor are disclosed. The spin valve sensor of the present invention comprises a first layer of ferromagnetic material and a second layer of ferromagnetic material. The second layer of ferromagnetic material has passive end regions separated by a central active region. A first layer of non-ferromagnetic material is positioned between the first and second layers of ferromagnetic material. A pinning layer is positioned adjacent to the first layer of ferromagnetic material such that the pinning layer is in contact with the first layer of ferromagnetic material. The spin valve includes a synthetic antiferromagnetic bias means extending over the passive end regions for producing a longitudinal bias in the passive end regions of a level sufficient to maintain the passive end regions in a single domain state. The synthetic antiferromagnets provide an increased pinning field and reduced side reading.
A preferred method for forming the spin valves of the present invention comprises depositing a first layer of antiferromagnetic material. A first layer of ferromagnetic material is deposited upon the first layer of antiferromagnetic material. A spacer layer is deposited upon the first layer of ferromagnetic material. A second layer of ferromagnetic material is deposited upon the spacer layer. A second antiferromagnetic layer is deposited upon the second layer of ferromagnetic material. A photoresist layer is deposited upon a central region of the second layer of antiferromagnetic material. The photoresist layer defines a central region in each of the second layer of antiferromagnetic material and the second layer of ferromagnetic material. The photoresist layer also at least partially defines first and second outer regions in each of the second layer of antiferromagnetic material and the second layer of ferromagnetic material. Contact material is deposited upon the first and second outer regions of the second layer of antiferromagnetic material. The photoresist layer is removed. Antiferromagnetic material is removed from the central region of the second layer of antiferromagnetic material.
The method of the present invention provides numerous advantages over the use of conventional techniques. A xe2x80x9cperfectxe2x80x9d interface is obtained between the exchange tab material and the free layer because there is no photo mask present to cause excess residue, shadowing or other complications. The exchange tabs and the electrical contacts are automatically aligned during the process. The milling of the exchange tab material can be stopped before reaching the interface between the exchange tab material and the free layer without removing any portion of the free layer, as long as the remaining thickness is less than the critical pinning thickness of the exchange tab material. The presence of the contacts allows the transfer curve characteristics of the sensor to be monitored during the mill, indicating if the mill time is adequate. Thus, the moment of the free layer is not reduced and there is not a problem with mismatched flux, stray fields or shield saturation. In addition, the process is easy to perform in high volume.
In an alternative embodiment of the method of the present invention, rather than removing antiferromagnetic material from the central region of the second layer of antiferromagnetic material, the central region is exposed to a reactive plasma, which alters the composition of the central region and ruins the exchange coupling.